The US space agency, Nasa, is getting ready to launch a piano-sized spacecraft to Pluto, the last unexplored planet in the Solar System.

New Horizons is set to lift off from Cape Canaveral, Florida, at 1824 GMT on Tuesday aboard an Atlas 5 rocket.

Despite being the fastest probe ever built, it will still take more than nine years to reach its distant target.

Anti-nuclear activists have staged small protests about the spacecraft's 33kg payload of plutonium fuel.

The $700m (£396m) New Horizons spacecraft will gather information on Pluto and its moons before - it is hoped - pressing on to explore the icy objects that reside in the region of space known as the Kuiper Belt.

We'll get our first glimpse of what the surface looks like and see whether there are craters, or volcanoes or frost or cracks

Dr Fran Bagenal, University of Colorado

This region, which lies beyond Neptune, consists of perhaps tens of thousands of icy objects spread out between 30 and 50 times the distance between the Earth and the Sun.

Some astronomers say Pluto is not a true planet at all, and should be classed instead alongside these icy relics from the formation of the Solar System.

"What we know about Pluto today could fit on the back of a postage stamp," said Colleen Hartman, a deputy associate administrator at Nasa. "The textbooks will be rewritten after this mission is completed."

Gravity assist

Stephen Lowry, of Queen's University in Belfast, commented that the mission could tell scientists much about Pluto and its moons.

"Dramatic differences on the surfaces may indicate that the system didn't form as a result of a single collision event. Perhaps some of the smaller members are gravitationally captured objects," he told the BBC News website.

If Nasa launches New Horizons before 3 February, the probe will be in position to swing by Jupiter, using the planet's gravity to pick up speed in a slingshot manoeuvre.

This will boost the probe's speed away from the Sun by nearly 4km/s, slashing the flight time to Pluto - and reducing the chances of a mission failure.

That boost will allow New Horizons to reach Pluto in July 2015. Otherwise, the journey will take until 2018 at the earliest.

New Horizons will fly by Pluto and its largest moon Charon on the same day. The spacecraft's seven instruments will gather information on Pluto's atmosphere and carry out detailed mapping of surface features on the planet and its biggest satellite.

Lots to learn

It will photograph the two small moons announced in November 2005 and check for rings around Pluto.

Some astronomers believe Pluto is not a true planet

Fran Bagenal, co-investigator on the mission, said: "We've never got a good view of what it looks like, even with the best telescopes - even with Hubble. It just looks like a fuzzy blob.

"We'll get our first glimpse of what the surface looks like and see whether there are craters, or volcanoes or frost or cracks," said Dr Bagenal.

After the Pluto encounter, it is up to Nasa to decide whether to grant the spacecraft an extended mission. Should this happen, mission scientists plan to send New Horizons to visit two Kuiper Belt Objects (KBOs) with diameters of 50km (30 miles) or more.

Scientists believe they can learn about the evolution of the Solar System by studying the Kuiper Belt since it possesses debris left over form its formation.

Window in time

"It provides for us a window 4.5 billion years back in time to observe the formation conditions of giant planets," said the mission's principal investigator, Alan Stern of the Southwest Research Institute in Boulder, Colorado.

Dr Lowry said studying these other Kuiper Belt Objects could shed light on how material interacted to form planetary objects in the early Solar System.

He added that scientists could also compare the surfaces of KBOs with those of comets photographed by previous space missions. This might provide clues to how comets evolve as they journey from the outer Solar System towards the Sun, he said.

For onboard electric power, the probe will convert heat from the decay of radioactive plutonium pellets into electricity for the spacecraft's systems and science instruments. Operating so far from the Sun means New Horizons cannot use solar panels.

Nasa said there was a 1 in 350 chance of a mishap that released plutonium around the launch site. Even so, it said, the chance of dangerous radiation exposure to workers and the public was low.

Pluto was discovered in 1930 by US astronomer Clyde Tombaugh while he was working at the Lowell Observatory in Flagstaff, Arizona.